Ventilation is employed to remove vapors arising from cooking oils and greases, which have been elevated above their vaporization temperature, as well as to remove odors associated with the preparation of food which may be considered objectionable in other parts of a building. Many known ventilating systems used in restaurants have been designed primarily as exhaust systems and depend upon the air within the building to move towards the ventilating systems and serve as a conducting vehicle to carry the fumes to be removed. Most of these exhaust systems employ some form of a canopy or hood, supported in a superior position relative to a cooking surface and provide a blower in communication with the hood to create a negative pressure, relative to the remainder of the building at an exhaust opening defined by the hood. These exhaust systems add an additional heat load to the air-conditioning or heating systems of such buildings. The additional heat load may represent as much as two-thirds of the total heat load of the building. Additionally, with such exhaust systems, which are employed to remove air laden with fumes and which contains an appreciable quantity of vaporized grease, there is a problem that the ambient air, which serves as the vehicle, is not at a sufficiently low temperature to effect condensation of the grease, prior to entry of the vaporized grease into a filter. Consequently, the vaporized grease escapes through the exhaust filter and subsequently condenses on the walls of the ducts in the exhaust system. This condensed grease constitutes a major fire hazard, particularly in restaurant buildings. Accordingly, many systems have adopted the use of a blower for both the exhaust and the inlet. However, due to the fact that air is then introduced from the outside, it is necessary to preheat the incoming air so as to prevent unduly lowering the temperature of the restaurant. This has been accomplished with some hood systems by the use of a perimeter supply plenum so that a laminar flow of air is introduced around the perimeter of the hood. This creates some discomfort for the cook standing directly below the flow of frigid air from the outside. Thus, in some cases, it has been found necessary to pretemper the cold air coming into the perimeter supply plenum defined by the hood.